This invention concerns thermoplastics compositions are articles made therefrom having gas and vapour barrier properties.
Thermoplastic materials are widely used in packaging because of their low cost and ease of forming into a variety of shapes. However, most thermoplastics materials suffer from the disadvantage of providing only a relatively poor barrier to gases and vapours. Poor gas barrier is a particular disadvantage in packaging oxygen sensitive materials such as foodstuffs which are to be stored unrefrigerated. Poor vapour barrier properties are a disadvantage when packaging materials which are sensitive to moisture vapour, for example foodstuffs and confectionery which lose condition when they become damp, and they are also a disadvantage when the packaged material includes flavouring components which diffuse through the packaging material with consequent loss of flavour.
A number of attempts have been made at improving the gas barrier properties of thermoplastics materials. GB-A-1136350, for example, proposes the use of circular platelike fillers with a ratio of diameter to thickness between 20:1 and 300:1 and a diameter of at most 40 xcexcm in polyolefin polymers selected from polyethylene, polypropylene, ethylene-containing copolymers containing at least 50 moles percent of ethylene, and polystyrene, the preferred amount of filler being 0.1 to 50 wt % of the total weight of filled polymer. Such filled polymer compositions are proposed to be used to manufacture films, for example for food packaging.
U.S. Pat. No. 3,463,350 is concerned with the production of moulded containers for packaging foodstuffs, the containers being made from mixtures of high density polyethylene (HDPE) and mica particles, for example by compression or injection moulding. Such containers are said to reduce the discolouration of so-called canned cornbeef as caused by oxygen compared with the use of similar containers made of HDPE filled with glass fibre or titanium dioxide instead of mica.
It has also been proposed in U.S. Pat. No. 4,528,235 to incorporate platelet filler particles with an average equivalent diameter of from 1 to 8 xcexcm, the maximum diameter being 25 xcexcm, and thickness of less than 0.5 xcexcm, into HDPE having a melt index of from 0.01 to 1.0 g/10 minutes at 190xc2x0 C. as measured by ASTM D-1238 to produce films having a thickness of from 10 to 100 xcexcm, with the intention of increasing the oxygen barrier of the films compared with films formed from unfilled HDPE.
Despite the apparent improvements in oxygen barrier resulting from these various hitherto proposed methods using lamellar fillers to impart oxygen barrier properties to polyolefins, even higher oxygen barrier properties have been sought. Thus U.S. Pat. No. 4,536,425 proposes increasing the gas barrier properties of polar thermoplastics resins, for example alkylene terephthalates, by blending the resins with mica flakes having a plurality of platelets using a shear force which effects delamination of the mica platelets and thereby a substantial increase in the aspect ratio of the flakes. Delamination of the mica flakes is said to occur as a result of shear forces generated during mixing of the resin with the mica being transferred to the mica flakes due to chemical bonding of silanol on the surface of the flakes to hydroxyl and carboxyl groups formed in the resin as a result of hydrolysis of ester groups in the resin at the temperature involved. This document goes on to state that the continuous delamination effect with polar thermoplastic resins is not encountered with non-polar resins such as unmodified polyethylene or polypropylene resins since non-polar resins do not normally form any appreciable adhesive covalent bonds with the mica platelet surfaces.
According to the present invention there is provided a method of making a moulding composition for forming an article having increased barrier to gases and/or vapours, the method comprising the step of mixing together a substantially non-polar thermoplastic resin together with a laminar filler, the laminar filler being capable of delaminating when the composition is subjected to high shear to increase the aspect ratio of the filler as it breaks down into platelets.
Compositions in accordance with the present invention have been found to provide a good barrier not only to oxygen but also to flavour molecules. A particularly preferred use of compositions in accordance with the present invention is as shoulders for toothpaste tubes. Whereas it is relatively easy to form the tubular part of such tubes from a rigid or flexible polymer laminate having barrier properties by the use of multi-layer structures including a layer of a polymer having barrier properties, for example ethylene/vinyl alcohol copolymers, this is not possible for the shoulder portion of such tubes. The result has been that the shoulders have had to be moulded from expensive thermoplastic resins or inserts of a barrier resin have had to be made and located within a shoulder having a low inherent barrier to flavour molecules, again leading to increased costs.
The non-polar thermoplastic resin is preferably a polyolefin resin, for example a polymer derived from one or more aliphatic or aromatic alkene, eg a polymer containing units derived from at least one of ethylene, propylene, but-1-ene and styrene. Examples of specific polyolefin resins which can be used include polyethylene, polypropylene, ethylene/propylene copolymers, ethylene, propylene/but-1-ene terpolymers, polyethylenes being particularly preferred by virtue of their good injection moulding characteristics. The polyethylene can be low density polyethylene (density 0.910 to 0.925 g.cmxe2x88x923), medium density polyethylene (density 0.925 to 0.950 g.cmxe2x88x923) or high density polyethylene (density 0.950 to 0.980 g.cmxe2x88x923). High density polyethylene is particularly preferred by virtue of its higher inherent barrier properties compared with lower density polyethylenes.
The platelet filler can be any of a variety of lamellar fillers provided the platelets delaminate under shear as is encountered when the filler is blended with the non-polar resin before injection moulding and more particularly when the mixture of filler and resin is subjected to injection moulding. Lamellar fillers include clays, mica, graphite, montmorillonite and talc. Talc is particularly preferred by virtue of its ease of delamination during shear.
The filler should have a structure consisting of platelets both before and after being subjected to high shear. As will be appreciated, in addition to delamination, subjecting such fillers to high shear also tends to reduce their effective diameter. However, despite a reduction in the effective diameter of the filler particles, high shear generally results in an increase in the aspect ratio of the individual filler particles.
Talc, being a naturally occurring hydrated magnesium silicate, is available in a variety of grades of greater or lesser purity. It has surprisingly been found that the ease of increasing the aspect ratio of talc when it is subjected to high shear in a non-polar thermoplastic resin appears to increase as the level of impurities within the talc decreases. Thus not only does it appear easier to delaminate the platelets of the talc, but the platelets themselves apparently resist fracture.
According to another aspect of this invention there is provided a composition for forming an article having increased barrier to gases and/or vapours, the composition comprising a substantially non-polar thermoplastic resin filled with platelets of talc having an aspect ratio of at least 5 and an average aspect ratio of between 16-30 and a CIE whiteness index of at least 40.
Whatever the mechanism by which the aspect ratio of some talcs is increased to a particularly high degree when they are subjected to high shear in non-polar thermoplastic resins, it has surprisingly been found that talcs which result in compositions in accordance with the invention having high values of CIE (Commission Internationale d""Eclairage) whiteness index are produced from talcs which delaminate relatively easily and resist fracture, that is they resist reduction in their diameter when sheared. Delamination of the talc has usually been found to occur if the CIE whiteness index of the composition after shearing is at least 40, and a significant increase in aspect ratio has usually occurred if the CIE index of the composition after shearing is at least 45. These CIE whiteness index values were determined for compositions containing 15 percent by weight of talc in high density polyethylene with no other filler present, the determination being in reflectance mode with UV light included and specular reflection excluded, the observer angle being 10xc2x0 and the samples being backed by a white tile.
Purer grades of talc are therefore generally preferred since it would appear that they lead to compositions in accordance with the invention which not only have good barrier properties but also high degrees of whiteness without the necessity to include a white pigment such as titanium dioxide.
Particularly preferred grades of talc for use in the present invention are sold by Richard Baker Horizon Group, England, under the Trade Mark xe2x80x9cMagsilxe2x80x9d, an especially preferred grade being xe2x80x9cMagsil osmanthusxe2x80x9d. 
Before being subjected to high shear, the filler particles preferably have an average particle diameter of not more than 100 xcexcm, more preferably not more than 50 xcexcm, and most preferably not more than 20 xcexcm. The particle thickness of the filler can also vary over a wide range, but it is preferably less than 10 xcexcm before being subjected to high shear, and more preferably less than 5 xcexcm thick.
The particularly preferred grade of talc referred to above as Magsil osmanthus typically has an average particle diameter of about 20 xcexcm and a thickness of about 2.5 xcexcm before being subjected to high shear.
The filler used in accordance with the present invention is reduced in thickness as a result of high shear, and this in general increases the aspect ratio of the filler particles even though thickness reduction is usually accompanied by a reduction in the average diameter of the filler particles. A typical increase in the aspect ratio of the filler particles is by a factor of at least 1.8, and preferably by at least three. For example, filler particles with an original aspect ratio of about 7 have had their aspect ratio increased to about 15 or more, for example to in excess of 21.
The high shear to which the filler particles are subjected in accordance with the present invention can be applied by various methods. It is particularly preferred to apply high shear during compounding prior to forming desired articles so that delamination of the filler particles is effected before forming the desired articles. Further delamination can also be effected during the forming step. It is generally preferred, however, to effect most of the delamination during the compounding operation, the preferred compounding operation being the use of a twin screw extruder or a Banbury mixer.
In addition to delamination of the filler particles, it is generally preferred to effect moulding of the filled resin under conditions which cause the filler particles to become oriented such that their larger face is substantially aligned with the surface of the mouldings. This is particularly effectively achieved by injection moulding the filled resin, and injection moulding has also led to a particularly effective delamination of the filler particles, thereby leading to an especially good barrier to flavour molecules.
Orientation of the filler particles can also be effected by extruding compositions in accordance with the present invention. The compositions can be extruded in various forms, for example films or tubes, having increased barrier properties. They can be extruded as a single web, or they can be coextruded with other layers on one or other side of a core layer formed from a composition in accordance with the present invention. For example films or tubes produced from compositions in accordance with the present invention can be used to form the body portion of toothpaste tubes, and a particularly preferred combination is of a tubular body portion made from a composition in accordance with the present invention in combination with a shoulder portion made from a composition in accordance with the present invention.
Although of particular value in the production of toothpaste tubes, it will be appreciated by those skilled in the art that the end use of the tubes can be for any purpose, but uses exploiting the particularly good barrier properties of compositions in accordance with the present invention are particularly preferred, especially after a forming operation in which the filler particles become aligned parallel to the surface of the formed articles produced therefrom.